Tens of millions of people in the Ganges Delta continue to drink groundwater that is dangerously contaminated with arsenic and manganese. We estimate that in Bangladesh, if consumption of contaminated water continues, the prevalence of arsenicosis and skin cancer will be approximately 2 million and 100,000 cases per year, respectively, and the incidence of death from cancer induced by arsenic will be approximately 3,000 cases per year4. Although less attention has been given to manganese, dangerous levels of manganes are also common in Bangladesh's groundwater. In their landmark survey of groundwater chemistry, the BritisH Geological Survey2 found that manganese and arsenic are the two contaminants that routinely exceed safe concentrations in Bangladesh, and recent epidemiological work has shown that manganese may damage the neurological function of Bangaldeshi children5. Bangladesh is an ideal field site for studying processes that mobilize toxic metals into the environment under geochemical reducing conditions. The anoxic groundwater conditions of Bangladesh contrast to the Ta Creek site of Project 2, where mining has exposed large quantities of minerals to rapid oxidation. In Bangladesh, construction of irrigated rice fields and ponds has introducing large inflows of anoxic, organic-ric water. This shift has occurred over the last forty years and, with the advent of irrigation pumping, the residence time of groundwater is now also on the order of decades to a century. Consequentially, the effects of these chemical alterations to recharge water are now moving through the groundwater system and driving biogeochemical process that can be observed to both mobilize and sequester toxic metals. In this proposed Superfund Basic Research Program (SBRP), our research will address how land use and groundwater transport control arsenic and manganese concentrations in drinking water. These questions lie z the intersection of hydrology and biogeochemistry and are key unresolved questions for understanding metal mobilization in the environment. Over the last decade, we have gained a nuanced understanding of the static geochemical characteristics of arsenic-contaminated aquifers and have characterized the rapid microbialmediated response to chemical perturbations in sediment incubation experiments. However, remarkably little known about basic aspects of hydrogeology that are vital for understanding the evolution of groundwater chemistry along flow paths. Although water-balance data exist from an agricultural perspective6, we do not know how the solute fluxes that drive manganese and arsenic mobility enter the aquifer, what patterns groundwater flow follows, or how solutes mix across different flow paths. Little is known about deeper groundwater flow, and indeed, basic issues such as the significance of regional flow7"9 and groundwater pumping 10~12 are still controversial. At our field area, we have recently quantified the seasonal hydrologic cycle by which aquifers are recharged and discharge. In this project, we will build on this water balance to characterize the locations and biogeochemical conditions where arsenic and manganese are mobilized as we as the subsequent transport of these toxins through the complex transient three-dimensional pattern of streamlines that deliver them to drinking-water wells. As part of our effort, we will work to advance two technologies that have the potential for widespread application. We will develop new methods for using sensors to monitor shifting biogeochemical conditions, and we will experiment with new methods for in-situ remediation of arsenic and manganese. A network of probes will be constructed to monitor, at high temporal resolution, changing soil conditions over hours, seasons and years. We will focus on practical issues of limiting drift and biofouling and will integrate the geochemical probes with our successful network of vadose-zone hydraulic sensors. Our in situ remediation experiments will focus on push-pull well-injection methods that use oxidizing agents to create "filters" around well screens to adsorb arsenic by coating aquifer sediments with precipitated manganese and iron oxides.